The Dutch supermajor is planning to spend $970 million over six years exploring for oil in four deepwater blocks off the continental shelf, roughly 250 kilometres south-southwest of Halifax.

“The primary objective of the proposed 3-D seismic survey is to identify potential drilling targets,” said a environmental assessment of the Shelburne Basin project prepared by LGL Ltd., an ecological research company in Toronto.

Shell is looking at old seismic data gathered from the four parcels to get a better handle on the geology.

“But due to the geological complexity of the Scotian Slope region, acquisition of 3-D seismic data is considered to be the most effective method to accurately map subsurface features in this area and, more appropriately, identify potential drilling locations,” said the 293-page report, released this week.

Once a potential drilling site is located, Shell plans to conduct a geohazard survey or use pre-existing survey data to look for, and thus avoid, “steep and/or unstable substrates or pockets of ‘shallow gas’ and seabed obstructions (man-made or natural),” the assessment said.

Shell is planning to conduct wide-azimuth surveys to examine the area in question. That is slated to last 65 to 120 days and take place from April to September. The ocean is 1,500 to 3,500 metres deep there.

“The 2013 survey is expected to involve four vessels, all towing air-gun source arrays, with the two outer vessels also towing streamers (that contain the receivers),” said the assessment.

The air guns will be activated sequentially every 10 seconds.

“This configuration is designed to enhance the efficiency of the operation and to provide a more complete image of the subsurface.”

The assessment looks at potential effects the survey could have on sea creatures.

“It is possible that leatherback sea turtles might exhibit temporary hearing loss or perhaps even permanent hearing damage if the turtles are close to the air gun,” it said, noting there are likely to be few of them in the area.

The noise should not have much impact on fish, the assessment said.

“While there is evidence for physical and physiological effects on all fish life stages, these effects were observed only when the subject was exposed in close proximity to the seismic noise source (i.e., metres) and sometimes could not move away from the seismic noise source (i.e., captive subject),” it said.

“In natural conditions, juvenile and adult fish stages can move away from a seismic source. While eggs and larvae do not have to ability to actively move away, a very small proportion of all the eggs and larvae in an area would be exposed to the seismic source at very close range.”

Shell’s survey work “is judged to have minor effects on commercial fisheries, over a short-term duration of (less than) one month and a geographic extent judged to be” 11 square kilometres to 100 square kilometres, said the assessment.

“Therefore, residual effects … on commercial fisheries are judged to be not significant. The level of confidence associated with this judgment is medium to high.”

During the seismic testing lookouts and underwater microphones will be used to detect marine mammals that may be sensitive to the air-gun noise, said the assessment. And the sound from the air guns will be ramped up over a half-hour period in order to drive creatures away from areas being surveyed.

The noise from seismic work is not predicted to have a significant impact on North Atlantic right whales, said the assessment.

“The level of confidence associated with this judgment is medium.”

There is a “minimal” risk one of the slow-moving survey ships could strike one of the endangered creatures, it said.

Blue whales, also endangered, could be disturbed by the air guns, but they are expected to avoid the noise, the assessment said.

Northern bottlenose whales could also be bothered by the sounds, but it is uncertain how many will be in the area during the period the survey is taking place.

“Numbers are expected to be low, given that this species is most likely to occur outside of the study area on the eastern Scotian Shelf in the Gully and adjacent submarine canyons. The study area does not contain any designated critical habitat for bottlenose whales and is not known to be an important feeding or breeding area for bottlenose whales.”

Air-gun noise is “judged to have minor to moderate disturbance effects on fin whales” over the short-term, but “residual effects related to disturbance are judged to be not significant,” said the assessment.

The noise is “judged to have negligible hearing impairment effects” on Sowerby’s beaked whales, it said, noting that the survey work may disturb them moderately.